This invention provides new polymeric ionomer compositions which have low haze. Low haze makes the compositions especially suited for use in clear packaging films, in addition to the other applications in which ionomers are utilized. This invention also provides a method for making these new polymeric compositions and for modifying the acidity of the compositions.
The compositions comprise ionomers which can be represented as the polymerization product of alpha-olefins having from two to eight carbon atoms, esters of alpha, beta-ethylenically-unsaturated carboxylic acids, metal salts of acrylic and methacrylic acid, and optional alpha, beta-ethylenically-unsaturated comonomers which impart some desired polymer property or properties, such as acidity and/or solvent resistivity. These ionomer compositions can easily be formed into films.
Ionomers which can be formed into films and methods of making ionomers are known in the art. Although these previously-known ionomers have similar chemical constituents to the ionomer compositions of this invention, the known ionomers have significantly different properties from the compositions of this invention. In addition, the known processes for making ionomers are also quite different from the method of making compositions of the present invention. Japanese patent number Sho 49-31556 to Iwami et al., dated Aug. 22, 1974, discloses a process for making ionomers which comprises saponifying copolymers of ethylene and alpha, beta-ethylenically-unsaturated carboxylate esters with a basic metal compound in aliphatic alcohol or in an organic solvent containing an aliphatic alcohol. The copolymer is homogeneously or heterogeneously dispersed in the alcohol solution. The saponified product can be further acidified to provide a composition having acid groups. Although the ionomers which have acid functionality are said to have low haze, no haze values are provided for ionomers which have no acid functionality.
Japanese patent number Sho 53-134591 to Harada et al., dated Nov. 24, 1978, discloses a film made by the process of Sho 49-31556 which is said to be useful for stretch-wrap applications. Their ionomer comprises a copolymer having 90-98 mole percent ethylene, 9.7 to 2.0 mole percent of an alkyl ester of an unsaturated carboxylic acid, 0 to 2.5 mole percent of unsaturated carboxylic acid, and 0.3 to 2.5 mole percent of a metal salt of an unsaturated carboxylic acid. It is stated that their film has very good mechanical, thermal, and optical properties, but the film is limited to having less than 9.7 mole percent ester because blocking occurs between film layers. In addition, the copolymer is limited to a maximum of 2.5 mole percent metal salt of an unsaturated carboxylic acid due to the viscosity of the copolymer being too high to allow processing of the copolymer. All copolymers in the films of the examples contain an unsaturated carboxylic acid component, which, the patent states, are used to adjust the modulus of elasticity and transparency of the film.
U.S. Pat. No. 5,218,057, issued to V. Kurkov and L. Theard on Jun. 8, 1993 and which is incorporated by reference in its entirety, teaches a process for saponifying an ethylene alkyl acrylate copolymer which comprises adding an aqueous solution of an inorganic alkali metal base to a molten ethylene-alkyl acrylate copolymer and mixing the alkali metal base and copolymer at a temperature sufficient for saponification to take place and at which the ethylene-alkyl acrylate copolymer remains in a molten or fluid state. Copolymers made by this process have lower tensile strength, lower gloss, and much higher haze than the copolymers of the present invention.
U.S. Pat. No. 4,638,034 to McClain, dated Jan. 20, 1987 and which is incorporated by reference in its entirety, claims a process for preparing an ethylene-acrylic acid copolymer salt which comprises saponifying an ethylene-alkyl acrylate copolymer in the molten state, in the absence of solvent or water other than by-product alkanol, with alkali metal hydroxide or alkaline earth metal hydroxide under non-static mixing conditions so as to thereby form alkanol and an alkali metal or alkaline earth metal salt of ethylene-acrylic acid copolymer, and separately recovering the alkanol and the salt.
U.S. Pat. No. 3,970,626 to Hurst et al., dated Jul. 20, 1976, discloses a copolymer of ethylene, alkyl acrylate or methacrylate, and an alkali metal salt of acrylic or methacrylic acid. Although this patent is mainly concerned with forming copolymers which form stable aqueous emulsions, the patent states that the copolymer can be extruded into films of good flexibility. Copolymers in the examples are formed using a batch autoclave. It is stated that about a one-fold excess of sodium hydroxide over what is theoretically required is used to convert the ester groups.
U.S. Pat. No. 4,042,766 to Tatsukami et al., dated Aug. 16, 1977 and which is incorporated by reference in its entirety, provides a method for preparing ionically cross-linked copolymers comprising melt-blending a copolymer comprising 1) ethylene and 2) at least one alkyl acrylate or methacrylate where the alkyl is selected from the group consisting of isopropyl or tert-butyl, with 3) at least one metal compound selected from the group consisting of acetates, formates, and oxides of zinc, magnesium, calcium, and sodium, and maintaining the molten blend at a temperature of about 200.degree. to 320.degree. C. The patent states that high mixing efficiency is desirable in the reaction equipment to assure uniform dispersion of the metal compound into the ester copolymer and to assure quick evaporation of the low molecular-weight byproducts, such as by melt-blending the components. Per the patent, adequate mixing was provided by a 20 mm-diameter single-screw extruder having a retention time of about one minute, as illustrated in Example 1 of that patent.
U.S. Pat. No. 3,789,035 to Iwami et al., dated Jan. 29, 1974 and which is incorporated by reference in its entirety, discusses three methods for acidifying an ionomer of a copolymer of ethylene and an ester of an alpha, beta-ethylenically-unsaturated carboxylic acid. The ionomer is made by saponifying a copolymer of ethylene and an ester of an alpha, beta-ethylenically-unsaturated carboxylic acid with a basic metal compound in a solvent containing an alcohol. The patent states that the alcohol is used to promote the reaction of the basic metal compound with the copolymer. The ionomer is then acidified by either 1) adding acid and replacing some of the basic metal with hydrogen; 2) melt-blending a polymer having acid groups with an ionomer; or 3) exchanging a non-alkali metal ion with the alkali metal ion on the ionomer which has been dispersed in a solvent.
U.S. Pat. No. 3,264,272 to Rees, dated Aug. 2, 1966, claims a composition comprising a random copolymer of an alpha-olefin having from two to ten carbon atoms, an alpha, beta-ethylenically-unsaturated carboxylic acid having from three to eight carbon atoms in which 10 to 90 percent of the acid is neutralized with metal ions, and an optional third mono-ethylenically unsaturated comonomer such as methyl methacrylate or ethyl acrylate. This copolymer is acidic due to the carboxylic acid groups present in the copolymer. U.S. Pat. No. 3,404,134 discloses the process for making these compositions, which comprises reacting an ethylene acrylic acid copolymer with a metal compound at a pressure between 100 and 10,000 psi and a temperature above the melt-point of the copolymer.
U.S. Pat. No. 5,189,113 to Muehlenbernd et al., dated Feb. 23, 1993, discloses a process for making ionically cross-linked copolymers of ethylene and alpha, beta-ethylenically-unsaturated carboxylic acids or alpha, beta-ethylenically-unsaturated comonomers donating carboxyl groups, such as anhydrides. This process requires reacting the copolymer with a solid metal compound in a mixing zone of a twin-screw extruder and subsequently pumping in water. The advantages for this process are said to be that no discoloration of the ionomer occurs because no corrosion of the twin-screw extruder occurs, and no specks of unreacted solid metal compound are found in the ionomer film.
U.S. Pat. No. 5,003,001 to Hasenbein et al., dated Mar. 26, 1991, claims a process for making ionically cross-linked copolymers of ethylene and alpha, beta-ethylenically-unsaturated carboxylic acids or alpha, beta-ethylenically-unsaturated comonomers donating carboxyl groups, such as anhydrides. This process reacts the copolymer with an aqueous metal salt solution in a first reaction zone at a temperature from 140.degree. to 180.degree. C. to form ionomer and completely devolatilizes the ionomer in a second, three-stage devolatilization zone at a temperature from 200.degree. to 270.degree. C. This process is said to provide odor-free ionomer film which is free of specks.
Although there has been much research in the area of ionomers, what has been lacking in the prior art is optically clear ionomers comprising copolymers of alpha olefins having two to eight carbon atoms, esters of alpha, beta-ethylenically-unsaturated carboxylic acids, and metal salts of alpha, beta-ethylenically-unsaturated carboxylic acids as well as methods of making those ionomers. This invention provides such compositions and methods.